The present invention relates to a piezoelectric resonator element and a method of manufacturing the same, and particularly to a method of manufacturing a thin film bulk acoustic resonator (hereinafter described as an FBAR) using an electroacoustic effect exhibited by a piezoelectric substance layer, and the FBAR.
Recently, with higher functionality and higher speed of portable telephones and PDA devices, there has been a stronger desire than ever for reductions in size and cost of high-frequency filters operating at a few hundred MHz to a few GHz which filters are included in the communication devices. A strong candidate for a high-frequency filter meeting this desire is a filter using an FBAR that can be formed by using semiconductor manufacturing technology.
FIGS. 6A and 6B show an example of a structure referred to as an air bridge type as a typical example of the FBAR (see for example, K. M. Lakin, “Thin film resonator and filters”, Proceedings of the 1999 IEEE Ultrasonics Symposium, (USA), Vol. 2, pp. 895-906 (hereinafter referred to as Non-Patent Document 1)). FIG. 6A is a sectional view. FIG. 6B is a plan view. FIG. 6A is a sectional view taken along a line A-A′ of FIG. 6B.
As shown in FIG. 6A, a structure 16 formed by sequentially forming a lower electrode 13, a piezoelectric substance layer 14′, and an upper electrode 15 is provided on a substrate 11 formed of high-resistance silicon or high-resistance gallium arsenide with an air layer 12 between the structure 16 and the substrate 11.
The lower electrode 13 is provided in a state of closing the air layer 12 and in a state of being extended in one direction on the substrate 11 (see FIG. 6B). The piezoelectric substance layer 14′ is provided over the substrate 11 in a state of covering the lower electrode 13. Further, the upper electrode 15 is provided in a state of at least a part of the upper electrode 15 being laminated on the lower electrode 13 over the air layer 12 with the piezoelectric substance layer 14′ between the upper electrode 15 and the lower electrode 13. This upper electrode 15 is extended in an opposite direction from the lower electrode 13, and is provided with a narrower width than the lower electrode 13 (see FIG. 6B).
A plurality of hole parts 17 in a state of reaching the air layer 12 are provided in the piezoelectric substance layer 14′ and the lower electrode 13 in an area outside the upper electrode 15. The air layer 12 communicates with an air outside the structure 16 via only the hole parts 17.
A part formed by laminating the lower electrode 13, the piezoelectric substance layer 14′, and the upper electrode 15 forms a vibrating part 18 of the FBAR. The lower electrode 13 is thus provided in contact with the air layer 12. Therefore, as with the upper electrode 15, the lower electrode 13 is formed with a boundary surface in contact with the air. Since the FBAR formed as described above has the vibrating part 18 provided above the substrate 11 with the air layer 12 between the substrate 11 and the vibrating part 18, the FBAR is easily mounted in such a manner as to be mixed with a compound monolithic microwave integrated circuit (compound MMIC) or a silicon IC. This feature suits needs for smaller size and higher functionality in the market.
Operation of the FBAR will be described in the following. When a temporally changing electric field is produced within the piezoelectric substance layer 14′ by applying an alternating voltage between the upper electrode 15 and the lower electrode 13, the piezoelectric substance layer 14′ converts a part of electric energy into mechanical energy in the form of an elastic wave (hereinafter described as a sound wave). This mechanical energy is propagated in a direction of film thickness of the piezoelectric substance layer 14′, which direction is a direction perpendicular to an upper electrode surface 15a and a lower electrode surface 13a, and is reconverted into electric energy. There is a specific frequency at which excellent efficiency is obtained in the electric energy/mechanical energy conversion process. When an alternating voltage having this frequency is applied, the FBAR exhibits a very low impedance.
This specific frequency is generally referred to as resonance frequency γ. Ignoring the presence of the upper electrode 15 and the lower electrode 13, the value of the resonance frequency γ is given by γ=V/(2t) as a first approximation, where V is the velocity of a sound wave in the piezoelectric substance layer 14′, and t is the thickness of the piezoelectric substance layer 14′. Letting λ be the wavelength of the sound wave, a relational equation V=γλholds, and hence t=λ/2. This means that the sound wave induced in the piezoelectric substance layer 14′ repeatedly reflects upward and downward between a boundary surface between the piezoelectric substance layer 14′ and the upper electrode 15 and a boundary surface between the piezoelectric substance layer 14′ and the lower electrode 13, and that a standing wave corresponding to half the wavelength of the sound wave is formed. In other words, the resonance frequency γ is obtained when the frequency of the sound wave causing the standing wave of half the wavelength of the sound wave and the frequency of the externally applied alternating voltage coincide with each other.
As an electronic device utilizing the very low impedance of the FBAR at the resonance frequency γ, a band-pass filter that has a plurality of FBARs combined into a ladder configuration and passes only an electric signal in a desired frequency band with a low loss is disclosed in the above-mentioned Non-Patent Document 1. In order to set a wider frequency passband in the band-pass filter, it is necessary to increase a difference between the resonance frequency γ of the FBAR and half of the resonance frequency. As means for this, there is a method of allowing each atom forming the piezoelectric substance layer 14′ to be moved by an external electric field more easily by applying a tensile stress to the piezoelectric substance layer 14′.
Such an FBAR is manufactured as follows. First, a sacrifice layer (not shown in the figure) is pattern-formed into a desired form on a substrate 11. Next, a lower electrode 13 is pattern-formed on the substrate 11 in a state of covering the sacrifice layer and extending in one direction. Next, a piezoelectric substance layer 14′ is formed on the substrate 11 in a state of covering the lower electrode 13. At this time, a tensile stress is applied to the piezoelectric substance layer 14′ by adjusting film-forming conditions.
Next, an upper electrode 15 is pattern-formed on the piezoelectric substance layer 14′ over the sacrifice layer in a state of extending in an opposite direction from the lower electrode 13. Then, hole parts 17 reaching the sacrifice layer are formed in the piezoelectric substance layer 14′ and the lower electrode 13 in an area outside the upper electrode 15. Thereafter an air layer 12 is formed by removing the sacrifice layer by wet etching that introduces an etchant from the hole parts 17.